Although two stroke internal combustion engines have been known for more than a century, they still have significant drawbacks and they have had limited application. There are heating problems normally experienced with two stroke internal combustion engines. Furthermore, the exhaust emissions from such engines include several atmospheric pollutants such as carbon monoxide, hydrocarbons and nitrogen oxides.
Carbon monoxide is a colourless, odourless, poisonous gas, slightly lighter than air. The presence of this noxious gas in exhaust emissions is the result of incomplete combustion of fuel with the carbon partly oxidized to carbon monoxide instead of being fully oxidized to carbon dioxide. This is due to insufficient oxygen in the combustion chamber. Most conventional combustion chambers respond adversely to over-supply of oxygen or to a "lean mixture" as it is normally called.
The presence of hydrocarbons in exhaust emissions also represents unburned and wasted fuel. Generally, the percentage of hydrocarbons is high in emissions from two stroke engines and this is particularly due to the nature in which the engines are scavenged. Although gaseous hydrocarbons at concentrations normally found in the atmosphere are not toxic, they are a major pollutant because of their role in forming photochemical smog.
Nitrogen oxides are produced when fuel is burned at very high temperatures in the presence of oxygen. Nitrogen oxides combine with hydrocarbons to form a complex variety of secondary pollutants called photochemical oxidants which contribute to the formation of smog. The presence of nitrogen oxides in exhaust emissions has become a major problem in all conventional two stroke internal combustion engines, particularly where efforts have been made to reduce carbon monoxide by burning lean mixtures. Some experimental units have been successful in burning lean mixtures, but the excess oxygen in the combustion chamber converts to nitrogen oxides, which in the past have only been able to be removed by installing an expensive catalytic converter.
Among the oxides of nitrogen forming air pollutants, nitric oxide (NO) and nitrogen dioxide (NO.sub.2) occur most frequently. Nitric oxide is a colourless, toxic gas formed from nitrogen and oxygen at high temperatures. It converts to nitrogen dioxide, an irritant and poison, in the exhaust of an internal combustion engine.
The formation of nitrogen oxides is a result of excess oxygen, combustion chamber temperatures above 1650 degrees Celsius and the dwell period of the piston at top dead center. The dwell period cannot be reduced in a conventional crankshaft engine. However, it has been found that if the zone of high temperature (i.e. above 1650 degrees Celsius) is removed from the location of excess oxygen, the generation of nitrogen oxides is substantially prevented.